Electronic switching system

ABSTRACT

An electronic switching system, particularly for a base band of video signals, comprises a diode bridge with a signal source and a load connected to respective ends of a main diagonal and a pair of normally blocked switching transistors connected between respective ends of a control diagonal and opposite terminals of a d-c supply. The two switching transistors are of opposite conductivity types and are cascaded to conduct simultaneously upon the application of an unblocking voltage to one of them, thereby causing a biasing current to flow in parallel through two pairs of bridge diodes in their forward direction. In the absence of this biasing current, the control diagonal is virtually shortcircuited by a low-resistance path including either a pair of serially connected ancillary diodes with grounded junction or two ancillary transistors, also of opposite conductivity types, which are cut off in the conductive condition of the switching transistors.

United States Patent Marchetti et al.

[ ELECTRONIC SWITCHING SYSTEM [21] Appl. No.: 241,280

[30] Foreign Application Priority Data Apr. 6, 1971 ltaly 22767/71 [52]US. Cl 307/257, 307/254, 307/255 [51] Int. Cl. H03k 17/00 [58] Field ofSearch 307/257; 328/308; 323/75 F [56] References Cited UNITED STATESPATENTS 3,159,751 12/1964 Bogdan, Jr. et al 308/257 X 3,389,272 6/1968Cherry 307/257 3,402,353 9/1968 Hubbs 328/208 X 3,179,817 4/1965Bounsall 307/257 X 3,449,596 6/1969 Milbcrger et al.. 307/257 3,573,5014/1971 Diehl 307/257 [5 7 ABSTRACT An electronic switching system,particularly for a base band of video signals, comprises a diode bridgewith a signal source and a load connected to respective ends of a maindiagonal and a pair of normally blocked switching transistors connectedbetween respective ends of a control diagonal and opposite terminals ofa d-c supply. The two switching transistors are of opposite conductivitytypes and are cascaded to conduct simultaneously upon the application ofan unblocking voltage to one of them, thereby causing a biasing currentto flow in parallel through two pairs of bridge diodes in their forwarddirection. In the absence of this biasing current, the control diagonalis virtually shortcircuited by a low-resistance path including either apair of serially connected ancillary diodes with grounded junction ortwo ancillary transistors, also of opposite conductivity types, whichare cut off in the conductive condition of the switching transistors.

6 Claims, 5 Drawing Figures PATENTEUNAR 12 I974 SHEET 2 (IF 2 1ELECTRONIC SWITCHING SYSTEM Our present invention relates to anelectronic switching system for selectively blocking and unblocking asignal path such as, for example, a channel carrying a base band ofvideo signals ranging up to about 5 MHz.

Such a switching system is useful, for instance, in periodically orrandomly monitoring the outputs of several television cameras at atransmitting station or in checking different program (e.g.,commercials) to be fed at certain times toa transmitter from a varietyof video tapes or other sources.

The general object of our invention is to provide an improved switchingsystem for the purpose set forth which allows a virtually instantaneouschangeover between open and closed circuit and, in the open-circuitcondition, sharply attenuates the signals of the blocked channel so asto avoid distortion of signals from another channel and to preventcross-talk between the several channels.

A more particular object is to provide a system of this character whichattains the aforestated desiderata with a minimum number of activecomponents (e. g., transistors) and which can be conveniently realizedwith integrated-circuit technique.

An electronic switching system embodying our invention includes aconventional rectifier bridge of the type using four diodes inrespective arms thereof, connected in two parallel branches of likepolarity between a first and a second bridge corner together defining acontrol diagonal; the junctions of the two serially connected diodes ofeach branch constitute a third and a fourth bridge corner togetherdefining a main diagonal for the passage of the controlled signals,these third and fourth corners being respectively connected to thesignal source and to a load such as, for example, a monitoring viewer.The bridge is biased, in accordance with our invention, by a circuitarrangement including a direct-current supply whose positive andnegative terminals are connected to the respective corners of thecontrol diagonal through a first and a second transistor of oppositeconductivity types (NPN and PNP) hereinafter referred to as switchingtransistors. These switching transistors are so poled as to pass, intheir conductive condition, abiasing current through the two branches ofthebridge'in the forward direction of all its diodes so that, upon asimultaneous reversal of the state of conductivity of these transistors,the flow of this biasing current is alternately blocked and unblocked.Such a reversal of the switching transistors can be brought about by asimple keying of the base voltage of one transistor (e.g., the firsttransistor) whose collector is connected to the base of the other (e.g.,the second) transistor so that the two transistors operate as a pair ofcascaded switching stages. Furthermore, the corners of the controldiagonal are bridged by electronic circuitry which responds to thebiasing current for establishing a low-resistance shunt path acrossthese corners in the nonconductive condition of the switchingtransistors.

Advantageously, the conductivity types of the switching transistors areso chosen that their emitters can be tied directly to the correspondingsupply terminals to which their bases are also joined via resistiveconnections so that base and emitter are at the same position if noextraneous voltage is applied thereto, the transistor beingsubstantially cut off under these conditions.

The first transistor, inserted between the negative supply terminal andthe bridge, should therefore be of NPN type while the second one is ofPNP type. The presence of resistors of like magnitude in the collectorleads of these transistors balances the bridge with reference to groundin both switching conditions.

The electronic circuitry substantially short-circuiting the controldiagonal in the open-switch condition may include two ancillarytransistors of opposite conductivity types interconnected in essentiallythe same manner as the switching transistors, i. e., a third (preferablyPNP) transistor inserted between the positive supply terminal and thefirst bridge corner and a fourth preferably PNP transistor insertedbetween the negative supply terminal and the second bridge corner. Withthese ancillary transistors so coupled to the switching transistors asto saturate when the latter are cut off, and vice versa, the shunt pathacross the control diagonal extends through-these ancillary transistorsand the d-c supply which is assumed to be of low internal impedance.

Any modulation by the blocked signal of the small reverse currentflowing through the bridge by way of these ancillary transistors in theopen-switch condition is, therefore, effectively prevented from reachingthe load.

In a simplified modification designed for signals of relatively lowamplitude (e.g., up to about :12 V) the ancillary transistors may bereplaced by resistors in combination with a pair of ancillary diodeswhich are serially connected across the control diagonal with a polarityopposite that of the bridge diodes so as to offer a high resistance tothe biasing current of the switching transistors, a common terminal ofthese ancillary diodes being connected to a neutral point (usuallyground) of the d-c supply. In the closed-switch condition, i.e., withthe switching transistors conducting, a potential difference of blockingpolarity corresponding to the small voltage drop across the bridge isdeveloped on the control diagonal so as to make the ancillary diodesvirtually impassable to low-amplitude a-c signals.

In the open-switch condition these ancillary diodes conduct andeffectively shunt to ground any residual signals traversing the diodesin the input-side branch of the bridge. In this case, therefore, theinternal impedance of the d-c supply is not involved.

The invention will be described in greater detail hereinafter withreference to the accompanying drawing in which:

FIG. 1 is a set of diagrams showing different operating positions ofconventional switching systems of the general character here envisaged;I

FIG. 2 shows another switching arrangement of the prior art;

FIG. 3 is a circuit diagram of a rectifier bridge and associated biasingmeans as utilized in a system according to our invention;

' FIG. 4 is a circuit diagram of an electronic switching systemembodying the invention; and

FIG. 5 is a similar circuit diagram illustrating another embodiment.

Diagram (a) of FIG. 1 illustrates the principle of a switching systemoperable to cut off the flow of alternating-current signals normallypassing from a source I to a load U. A switch 1, closed during signaltransmission, is opened to break the circuit while another switch 2 inshunt with the load is simultaneously closed to leak off residualoscillations which may be transmitted (e.g., capacitively) over the opencircuit. This alternate position has been illustrated in diagram (b).Switches 1 and 2 may be constituted by purely electronic elements suchas transistors to permit a rapid changeover between open and closedcircuit; in the frequency range here contemplated (up to about MHZ),however, the maximum signal attenuation in the position of diagram (b)generally is only about 50 dB. Also the grounding of the output lead byshunt switch 2 prevents the multipling of several channels to that leadfor selective transmission of their signals to the load. This latterdrawback is remedied in the modified prior-art system of diagram (c)where a third switch 3 lies beyond shunt switch 2 in series with switch1, opening and closing simultaneously therewith.

With relatively complex circuitry including eight transistors, such asystem may attain an attenuation up to about 75 or 80 dB.

FIG. 2 shows the multipling of several switching circuits S and S" forthe selective feeding of signals from respective sources I and I" to acommon load U. Each of these circuits is generally similar to that ofdiagram (c) OF FIG. 1, comprising a shunt switch 2', 2" and a seriesswitch 3, 3" beyond the latter; the input switch (1) has not been shownand could in fact be omitted.

The system shown in FIG. 2 is inherently unbalanced and, unless one ofthe switches 3' and 3" is always closed, will develop a varying d-ccomponent resulting in a distinct voltage jump upon the closure of oneof its signal paths.

FIG. 3 shows one of the basic components of our improved switchingsystem. This component is essentially a rectifier bridge with corners A,B defining a main diagonal end corners C, D defining a control diagonal.The four arms of the bridge are constituted by respective diodes d d andd d all oriented with their anodes toward junction D and with theircathodes toward junction C. Upon the application of a negative biasingvoltage V to a terminal E, connected to corner C through a seriesresistor r and a positive biasing voltage +V of like magnitude to aterminal F, connected to corner D via a series resistor r a biasingcurrent flows through two parallel branches respectively consisting ofdiodes d d and diodes d d... The diode junction A of the first branch,on the input side of the bridge, is connected to the signal source Iwhereas the correspondingjunction B of the second branch, on the outputside, leads to the load U. The flow of biasing current through resistorsr and r renders the bridge conductive for the passage of a-c signalsfrom source I to load U. Such conduction, however, is blocked upon theapplication of a positive voltge +V to terminal E and of a negativevoltage V., of preferably the same magnitude to terminal F.

The two branches d d and d a of the diode bridge shown in FIG. 3 thusperform the functions of series switches 1 and 3 in diagram (c) of FIG.1 but cannot replace the shunt switch 2 thereof. The system according toour invention, as described hereinafter with reference to FIGS. 4 and 5,provides an equivalent for the latter switch by electronic circuitryestablishing a virtual short circuit between junctions C and D in theabsence of the aforementioned biasing current, this short circuit beingbroken in response to the flow of that current.

In FIG. 4 we have shown a negative bus bar 11 and a positive bus bar 12maintained at l2V and +l2V, respectively, by a d-c power supply notfurther illustrated. Bus bar 11 is connected to the emitter of a firsttransistor Tr, whose base is joined to that bus bar through a resistor rand is connectable through a resistor r, and a manual or electronicswitch T to a lead 13 of relatively positive potential (ground); thecollector of transistor Tr is connected to bus bar 12 through a voltagedivider consisting of two resistors r and r whose junction is tied tothe base of a second transistor Tr having its emitter directly joined tobus bar 12. Transistor Tr,, which is of NPN conductivity type, andtransistor Tr which is of the opposite conductivity type PNP, areserially interconnected via the aforedescribed biasing circuit includingresistors r r and diode pairs (1,, d and d d With switch T open, thebases of both switching transistors Tr Tr are at the potentials of theirrespective emitters so that their resistance is very high; closure ofswitch T causes the saturation of transistor Tr, whose collector currentinstantly drives the base of transistor Tr relatively negative, i.e., toapproximately ground potential if resistors r and r., are substantiallyidentical, so that transistor Tr also saturates and passes the biasingcurrent through bridge 41,- d.,. This is the condition represented inFIG. 3 by the application of voltages V and +V to the respective biasingterminals E and F which in FIG. 4 are constituted by the collectors ofthe two switching transistors.

Another voltage divider, connected between point F (i.e., the collectorof transistor Tr and bus bar 12, consists of two resistors r and r which(like resistors r and r.,) are of large magnitude compared with biasingresistors r and r The junction of resistors r and r is connected to thebase of a further PNP transistor Tr whose emitter is directly energizedby bus bar 12 and whose collector is tied to bridge corner C. In ananalogous manner a fourth transistor Tr, of NPN type has its emitterconnected to bus bar 1 l and its collector joined to bridge corner D;the base of transistor Tr, is connected to the junction of a resistor rwith a diode d constituting with a further resistor r a voltage dividerinserted between bus bars 11 and 12. Through a diode d positionedback-to-back with diode d the junction of the latter diode with resistorr is connected to point E, i.e., to the collector of resistor Tr,. Inthe nonconductive condition of switching transistor Tr therefore, thesmall current flowing through tbe large resistors r, and r as well asdiode d biases the base of ancillary transistor Tr., to substantiallyground potential so that this ancillary transistor is saturated andapplies negative potential (V., in FIG. 3) to bridge corner D. At thesame time, the collector current drawn by transistor Tr, throughresistors r r r biases the base of ancillary transistor Tr also toapproximately ground potential, the combined resistance of saturatedtransistor Tr, and resistor r being negligible compared with that of thetwo substantially identical resistors r and r Transistor Tr thereforesaturates and applies to bridge comer C a positive voltage as indicatedat +V in FIG. 3. Under these conditions, the four diodes d d of thebridge are strongly reverse-biased so as to block conduction betweenjunctions A and B. Since the impedance of the shunt path from junction Dthrough transistor Tr current supply :1 2V and transistor Tr to junctionC is low compared with that of the reverse-biased diodes d and 41,, anysignal components clearing the diodes d, and d at this time will beseverely attenuated before reaching the load U. Upon the subsequentclosure of switch T to saturate the switching transistors Tr, and Trancillary transistors Tr and Tr, are cut off so that the shunt path isopen and signals pass normally from source I to load U.

[n the modified system shown in FIG. 5, ancillary transistors Tr and Tr,have been replaced by two resistors r r' whereas two ancillary diodesd,, d' are connected in series between points C and D; these ancillarydiodes, whose junction is grounded, have a polarity opposite that ofdiodes d d so that their presence does not significantly affect themagnitude of the biasing current flowing through diodes d d, uponclosure of switch T. The voltage drop generated by this biasing currentbetween points C and D back-biases the ancillary diodes d, and d' sothat signals not exceeding a peak-to-peak voltage of about 2.5 V cannotpass through them. Upon the opening of switch T, residual signalcomponents traversing the diodes d, and d are shunted to ground throughdiodes d and d';.

A diode d between point E and resistor r prevents the grounding ofvoltage divider r r through that resistor and is balanced bp a similardiode d between point F and resistor r The system according to ourinvention introduces an attenuation of greater than 95 dB for signals upto about 5 MHz in the open-switch condition, with extremely shortswitchover periods on the order of 100 nsec.

We claim:

1. An electronic switching system comprising:

a rectifier bridge including four diodes in respective bridge armsforming two parallel branches of the same polarity between a firstbridge corner and a second bridge corner, said branches consisting eachof two bridge arms in series with junctions constituting a third and afourth bridge corner, said third and fourth corners being respectivelyconnectable to a source of alternating-current signals and to a load;

biasing means for said bridge including a directcurrent supply with apositive and a negative supply terminal, a first transistor of oneconductivity type inserted between one supply terminal and said firstcorner, and a second transistor of opposite conductivity type insertedbetween the other supply terminal and said second corner, saidtransistors being so poled as to pass in their conductive condition abiasing current through said branches in the forward direction of allsaid diodes;

control means for simultaneously reversing the state of conductivity ofsaid first and second transistors, thereby alternately blocking andunblocking the flow of said biasing current, said first transistorhaving an input electrode connected to said control means and an outputelectrode connected to an input electrode of said second transistor; and

electronic circuit means responsive to said biasing current forestablishing a low-resistance shunt path between said first and secondcorners in the nonconductive condition of said first and secondtransistors, said electronic circuit means comprising a third transistorof said one conductivity type inserted between said one supply terminaland said second corner, said third transistor having an input electrodeconnected to an output electrode of said second transistor, and a fourthtransistor of said opposite conductivity type inserted between saidother supply terminal and said first corner, said fourth transistorhaving an input electrode connected to the output electrode of saidfirst transistor.

2. A switching system as defined in claim 1 wherein said first andsecond transistors each include an emitter connected to the associatedsupply terminal, a base constituting its input electrode and a collectorconstituting its output electrode, said biasing means further includinga first resistive connection from the base of said first transistor tosaid one supply terminal, a second resistive connection from the base ofsaid second transistor to said other supply terminal, a first seriesresistor between the collector of said first transistor and said firstcorner, and a second series resistor of substantially the same magnitudebetween the collector of said second transistor and said second corner.

3. A switching system as defined in claim 2 wherein said first andsecond transistors are of NPN and PNP type, respectively.

4. A switching system as defined in claim 1 wherein the connectionbetween said output electrode of said first transistor and said inputelectrode of said fourth transistors includes two further diodesconnected backto-back and having a junction connected to one of saidsupply terminals.

5. A switching system as defined in claim 1 wherein said inputelectrodes are bases and said output electrodes are collectors, saidfirst and fourth transistors being of NPN type and having each anemitter tied to said negative supply terminal, said second and thirdtransistors being of PNP type and having each an emitter tied to saidpositive supply terminal.

6. A switching system as defined in claim 1 wherein said four diodes arepoled to pass current from said second comer to said first corner, saidfirst transistor being of NPN type and inserted between said firstcorner and said negative supply terminal, said second transistor beingof PNP type and inserted between said second corner and said positivesupply terminal.

23 3 "UNI'IIIED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 307960894 Dated 12 March 1974 Inventor) ohnrramo mum: It :1 (Albertomoment) It is certified that error appears in the ahove-ioentifiedpatent and that said Letters Patent are hereby corrected as shown below:

In the headi g, line [751 for the com! inventor:

first Alter-to" read Alberto and sealed this 9th day of 1974.-

- Attestz V McCOY M. GIBSON, JR. C. MARSHALL DANN I Attesting OfficerCommissioner of Patents

1. An electronic switching system comprising: a rectifier bridgeincluding four diodes in respective bridge arms forming two parallelbranches of the same polarity between a first bridge corner and a secondbridge corner, said branches consisting each of two bridge arms inseries with junctions constituting a third and a fourth bridge corner,said third and fourth corners being respectively connectable to a sourceof alternating-current signals and to a load; biasing means for saidbridge including a direct-current supply with a positive and a negativesupply terminal, a first transistor of one conductivity type insertedbetween one supply terminal and said first corner, and a secondtransistor of opposite conductivity type inserted between the othersupply terminal and said second corner, said transistors being so poledas to pass in their conductive condition a biasing current through saidbranches in the forward direction of all said diodes; control means forsimultaneously reversing the state of conductivity of said first andsecond transistors, thereby alternately blocking and unblocking the flowof said biasing current, said first transistor having an input electrodeconnected to said control means and an output electrode connected to aninput electrode of said second transistor; and electronic circuit meansresponsive to said bIasing current for establishing a low-resistanceshunt path between said first and second corners in the nonconductivecondition of said first and second transistors, said electronic circuitmeans comprising a third transistor of said one conductivity typeinserted between said one supply terminal and said second corner, saidthird transistor having an input electrode connected to an outputelectrode of said second transistor, and a fourth transistor of saidopposite conductivity type inserted between said other supply terminaland said first corner, said fourth transistor having an input electrodeconnected to the output electrode of said first transistor.
 2. Aswitching system as defined in claim 1 wherein said first and secondtransistors each include an emitter connected to the associated supplyterminal, a base constituting its input electrode and a collectorconstituting its output electrode, said biasing means further includinga first resistive connection from the base of said first transistor tosaid one supply terminal, a second resistive connection from the base ofsaid second transistor to said other supply terminal, a first seriesresistor between the collector of said first transistor and said firstcorner, and a second series resistor of substantially the same magnitudebetween the collector of said second transistor and said second corner.3. A switching system as defined in claim 2 wherein said first andsecond transistors are of NPN and PNP type, respectively.
 4. A switchingsystem as defined in claim 1 wherein the connection between said outputelectrode of said first transistor and said input electrode of saidfourth transistors includes two further diodes connected back-to-backand having a junction connected to one of said supply terminals.
 5. Aswitching system as defined in claim 1 wherein said input electrodes arebases and said output electrodes are collectors, said first and fourthtransistors being of NPN type and having each an emitter tied to saidnegative supply terminal, said second and third transistors being of PNPtype and having each an emitter tied to said positive supply terminal.6. A switching system as defined in claim 1 wherein said four diodes arepoled to pass current from said second corner to said first corner, saidfirst transistor being of NPN type and inserted between said firstcorner and said negative supply terminal, said second transistor beingof PNP type and inserted between said second corner and said positivesupply terminal.